Frozen saline soils of the Arctic coast: their distribution and engineering properties

Transcription

1 Permafrost, Phillips, Springman & Arenson (eds) 23 Swets & Zeitlinger, Lisse, ISBN Frozen saline soils of the Arctic coast: their distribution and engineering properties A. Brouchkov Research Center for North Eurasia and North Pacific Regions, Hokkaido University, Sapporo, Japan ABSTRACT: Frozen saline soils are widely distributed along the Arctic coast and have special engineering properties. They freeze at lower temperatures and contain more unfrozen water than other frozen soils. Frozen saline soils differ in their origins and chemical compositions. The origin of the salinity is related to marine sedimentation. However, continental processes are also involved. Changes in salinization alter creep capabilities of soil, and data on long-term creep is presented. Frozen marine deposits containing chloride ion have the lowest bearing capacity. Ice appears to be a bearing factor. Bearing capacity of fine-grained soils in a range of temperature and salinity was studied. Statistical data on construction along the Arctic coast of Russia were analyzed. 1 INTRODUCTION Frozen saline soils are among the most common features of the Arctic coast (Velli 198, Hivon & Sego 1993). They are also distributed in other regions, such as Central Siberia, where continental salinization is caused by predominance of evaporation above precipitation and is characterized by prevalence of the sulphate and carbonate ions (Brouchkov 1998). It has been shown that frozen saline soils have special properties and are characterized by low bearing capacity (Velli 198). They occupy the position between frozen and unfrozen soils because they freeze at lower temperatures and contain more unfrozen water than the same soils without salts. Frozen saline soils are distinguished by their physical characteristics, deformability, and other properties. Frozen saline soils are identified as those containing.5% by weight of soluble salt compared to a dry soil. The definition is based on the measurable change of mechanical properties; an example is illustrated in Fig. 1. Salinization is the accumulation (as an accumulated mass or quantity formed) of salts in the soil (Oxford English Dictionary 1989); salinization is also a translation of the same term from Russian. The term salinization (salt weight content in 1 gram of dry soil, %, D sal ) from our point of view is preferable for use because the salt content of pore water ( salinity ) of the same soil varies with temperature. The geotechnical conditions of the Arctic coast are complex due to low temperatures and wide distribution of saline and ice-saturated soils. Construction practice could be observed in small settlements: some of them have a long history. Construction of these settlements began in the 193s and 194s. The so-called first principle of construction is used in territories with continuous distribution of frozen soils, i.e. the preservation of the frozen condition. This principle has obvious advantages in comparison to the Strength, kpa Salinization, % Figure 1. Long-term strength of adfreezing for concrete and marine sand at 3 C. principle of preliminary thawing (principle two). In the majority of cases, frozen saline soils on the Arctic coast are characterized by continuous long-term deformations. This was observed in a group of houses in Amderma, Dikson, Tiksi and Pevek; these houses continued to deform over a period of 2 years. Supervision of over 14 buildings was carried out in Tiksi. Design calculations should be made for both the bearing capacity and deformations. However, in practice this certain requirement is not carried out due to absence of experimental work. This is one of the common problems of design. 2 OCCURRENCE AND DISTRIBUTION OF FROZEN SALINE SOILS ON THE ARCTIC COAST The formation of frozen saline soils occurs during the process of sedimentogenesis, through the redistribution of water and salts during the diagenesis of deposits and their freezing. In general, the origin of the salinization is related to seashore processes. Sea incursions cause thawing of frozen deposits under the sea and subsequent saturation of pore solution by sea salts. These play a major role in the formation of saline soils. 95

2 Table 1. Average salinization of frozen soils along the Russian Arctic coast. Area Sampling site Salinization, % European part Haipudir Bay.3 of Russia Amderma Kolguev island.2.8 Pechora river.1.2 Western Siberia Northern Yamal.5 2. Southern Yamal.2.5 Dikson.5 1 Gidan Eastern Siberia Chukotka.2 3 Pevek.1.5 Lena river.1 1 Alazeya river.2 1. The marine, glacial-marine, shallow-water, and lagoon deposits prevail among the soils. The salts within pore solutions are partially taken out upon freezing, and partially redistributed. Salt moves together with water toward the freezing front in clay deposits, and, by contrast, away from the freezing front in sand (Brouchkov 22). Basically, frozen saline deposits of up to 1 2 m in depth along the Arctic coast contain salts within the limits of.5 2% and belong mainly to the marine type of salinization with prevalence of chloride ion above all (Table 1). The active layer, except for some slope and coastal sites, is not saline (Brouchkov 1998). Formation of frozen saline fine-grained soils is a part of sedimentary process in cryolithozone. The distribution of salts is related to the structure of deposits and is determined by conditions at formation as well as by the subsequent processes in the frozen state. As field materials show, the marine type of salinization is characteristic of coast areas with absolute altitudes of 15 2 m above sea level, and north of the border of thawed frozen soils during the climatic optimum of the Holocene (Fig. 2). The continental type of salinization is common in dry areas of modern river, lake and slope sedimentation. The mixed type of salinization is typical for the land sea zone of interaction and is distributed throughout the area, where salts of both sea and continental origin contribute to the salinization of deposits (Brouchkov 22). The uniform distribution of salinization or its increase with depth is observed in most cases (Fig. 3). The non-uniform distribution of salinization with depth can be caused by conditions of formation. Saline chilled soils and cryopegs are common almost anywhere on the Arctic coast. An increase in salinization with depth (Fig. 3) is most likely caused by the process of salts being lost from the upper layer of permafrost due to the migration of salts and salts moving out in the active layer. There is Figure 2. Distribution of frozen saline soils in the Arctic: -marine type -continental type. Depth, m Salinization, Dsal, % n.1 n.2 n.3 n.4 Figure 3. Distribution of salts in frozen deposits of the third marine terrace in the Amderma area, Kara coast; numbers represent boreholes. 96

3 seasonal movement of unfrozen water carrying salts in frozen soils, which is directed according to a gradient of seasonal temperature: upwards in the winter, downwards in the summer (Brouchkov 2). Gradual removal of salts from upper permafrost occurs in natural conditions, and sections of older deposits of high marine terraces in Amderma area (Kara sea) are characterized by this salt distribution with depth. Therefore, the salt content of frozen soils is not stable. Salt transfer should be taken into account in design. The temperature mode and the heat balance of saline soils is special because of their thermal properties: high unfrozen water content and lower thermal conductivity. 3 MAJOR FEATURES OF COMPOSITION OF FROZEN SALINE SOILS The chemical and mineral composition of the frozen saline soils and grain size are similar to the composition of other frozen soils of the Arctic coast. The values of salinization vary in the Russian Arctic (Table 1). Organic substances are characteristic of saline soils in relation to low temperatures of sedimentation and fast freezing of the deposits. Chemical composition of pore waters (Velli 198, Aksenov & Brouchkov 1993) is characterized by the constancy and the dominance of sodium chloride, in case of marine type of salinization. The prevalence of sulphur, carbon acid, and calcium ions is typical for the frozen saline alluvial and other deposits of the Central Siberia ( continental type). Salinization is normally increased with the increase in ice content, which is related to the replacement of pore solution by ice during freezing. The frozen saline soils of the Arctic coast are usually coastal deposits, and fine-grained composition is a typical feature. Insufficient sorting of material, low density and sand dust are observed in the saline deposits. An essential characteristic of the structure of saline soils is their heterogeneity: a mixture of frozen (ice inclusions) and unfrozen (saline mineral parts with unfrozen water) constituents is present in the structure. Unfrozen water content determinates the bearing capacity (Tsytovich 1975, Williams & Smith 1989); therefore, frozen saline soils are characterized by the relative weakness. 4 MECHANICAL PROPERTIES OF FROZEN SALINE SOILS Mechanical properties of frozen soils depend on salt content (Fig. 1). The long-term creep properties of saline frozen soils have not been studied in depth (Velli 198, Brouchkov 1998). Strains of frozen saline soils of the marine type under constant uniaxial stress and at other conditions, as well as strains undergoing long-time deformation (more than 12 years) were examined. Three stages of deformation were observed: the stage of a damping creep, followed by the stage of a secondary creep, and the final stage of a progressing creep, which was typical for sand only at salinization of.3.2%. Passage to the secondary creep happened at the strain of 4 7%, and to the stage of increasing creep rate at strain of 8 11%. However, the stages were not observed for temperatures from 2 to 4 C for frozen saline silt (D sal.9%;.2; and W.36.4). Damping creep was observed up to strain of 2%. A decrease in creep rate with time is a characteristic feature of the frozen saline soils. A value of strain, for example 2% could be considered as the criterion of failure in such a case. By this feature of creep the frozen saline soils differ from non-saline frozen soils, where all three stages of creep are observed. There is also a difference between ice-saturated soils, where creep with the constant rate predominates. An increase of water content decreases the strains (Fig. 4). Therefore, ice plays a bearing role, and the value of strength of ice-saturated frozen saline soils is larger. Stresses of frozen saline soils are smaller with a reduction of grain size of soil (Fig. 5). Deformations are increased with salinization and temeratures according to the increase in the unfrozen water content. The type of salinization plays an important role: silt containing Na 2 SO 4 is about 1.5 times stronger than silt with NaCl on average (Fig. 6). One of the major problems of design calculations is the prognosis of long-time deformations of frozen saline soils. The engineering theories of the creep, based on a non-linear dependence between the rate of a creep and strain are simple and common (Vyalov 1986). We carried out long-term (duration of more than 12 years) experiments at the constant temperature and loads in the underground laboratory of Amderma permafrost station, which is located at the depth of 14 m Strain, % W=.3 W=.5 W= Time, hours Figure 4. Strains of marine sand at 2 C, salinization D sal.5% under uniaxial stress of.1 MPa and water content (W) of.3,.5 and.7. 97

4 Strength, MPa Sand Sandy silt Silt Figure 5. Long-term strength of frozen saline soils under constant uniaxial stress at 2 C, salinization D sal.2%. Strength, MPa Sodium chloride Calcium chloride Sodium sulphate Figure 6. Long-term strength found in the ball test of marine silt at 2 C, salinization D sal.5%; chemical composition of salts is different. Strain, % S1 S2 S Days Figure 7. Long-term strains of marine frozen saline silt under constant uniaxial stress of.1 MPa, at 3 C; salinization D sal.5%; specimens are numbered. underground. The temperature oscillations did not exceed.3 during the period of observation. The long-term creep of saline silt is characterized by a decreasing rate (Fig. 7). Redistribution of ice and some thickening of ice lenses took place in frozen saline samples during the experiments. We have found that the best correlation of calculated and experimental data has been achieved by using the Vyalov s formula (1986). 5 THE USE OF FROZEN SALINE SOILS AS BASES AND STATISTICS OF DAMAGES In the majority of cases frozen bases in the Arctic coast are susceptible to slow deformations. The problem with design is the overestimation of bearing capacity of frozen saline soils, particularly of the marine type of salinization. Phase transfers of frozen saline soils are in the range of negative temperatures, the period and depth of thawing increase, and this is often not taken into account. Consequently, deformations of the bases occur. Other reasons for deformations should be also considered. An increase in soil temperature in many settlements of the Arctic coast has been occurred after construction. For example, the mean annual soil temperature is about 7 inside, but 12 C outside of the settlement in Tiksi; it is 3 inside, but 4.5 outside of the settlement in Amderma. Thus, the bearing capacity of soils is normally overestimated before construction begins because the temperature changes were not considered. Snowdrift on the Arctic coast is a significant problem since snow cover acts as a warming agent. Some buildings in small settlements are placed in dangerous proximity to the sea. Numerous examples of destructions, caused by the thermoabrasion are known in Varandei, Amderma, Harasavei, and other places. As the result of inspection of about 2 buildings it has been established that the main reason for deformations is underestimation of bearing capacity of frozen soils and change of temperature mode after the construction, mostly because of thermal influence of buildings or wastewaters in the base. A poor management and departmental differences in the construction policy resulted in the significant problems. The damage to buildings from deformations along the Arctic coast is very extensive and has costs of at least few billions of rubles, valued in 199. Repair and restoration of buildings cost about 25 1% of the initial price of construction in the Russian Arctic. The number of deformed objects reaches 25 5% of their total number. Some typical examples follow. 1. Apartment building in Amderma has a ventilated basement (Fig. 8). An increase in the mean soil temperature occurred from 3 C up to 1.5 C so the building began to deform. 2. A high school in Amderma is a 3-storeyed, brick building on piles. Marine sandy silt is in the base; salt content increases with depth and reaches.6.9% at depths of 4 9 m. The design engineers of this building have not taken into account the salinization of soils. The building has many cracks. 98

5 Table 2. Statistic of damages in areas of frozen saline soils distribution. Town Total deformation rate, % Amderma, Kara sea coast Dikson, Kara sea coast Tiksi, Laptev s sea coast Pevek, Yakutsk, Lena river valley Table 3. Results of tests of concrete piles in frozen saline deposits by static loading in Amderma. Figure 8. Damaged apartment building, Amderma town, Kara Sea coast. Test Soils 1 Sand up to 4 m, then sandy silt; T 2 C, salinization.1% Sandy silt with T 2 C, salinization.1.2%, water content 3 5% Sandy silt, T 3.5 C, salinization.6.9%, water content 3 5% 2 3 Bearing capacity according CNR, kn Bearing capacity according tests of piles, kn The incorrect estimation of the bearing capacity of the bases is among the major reasons for the occurrence of deformations. Research has revealed a significant divergence between experimental data on strength of the frozen saline soils (R) and their resistance to shearing load (Raf) and data recommended by Constructions Norms and Regulations (CNR) The CNR were applied in the design of all settlements on the Russian Arctic coast. The CNR include mean values of strength for all types of salinization. Confirmation of the data obtained in the laboratory has been obtained by pile testing in Amderma (Table 3). It has been established by laboratory tests that the frozen saline soils in the Arctic coast of Russia have extremely low strengths. The recommended bearing capacity values are given in Table 4. The study of the frozen saline soils has resulted in a number of recommendations: Figure 9. Damaged boiler building, Amderma town. 3. Deformations are observed in one building located on 32 Polar Street in Amderma. This heavy brick 4storey building has some cracks in outside walls. The base is frozen saline (up to 1%) sandy silt. 4. The boiler building on Lenin Street in Amderma (Fig. 9) has deformations exceeding 1.5 m due to long-term creep and an increase of soil temperatures. Catastrophic deformations were also observed in other buildings of the Arctic settlements, power lines and heat pipelines in Dikson, and the sewer networks of Amderma. Due to flooding by the subpermafrost saline waters the construction of 48-room apartment building and kindergarten in Amderma and a 125-room condominium in Pyramid (Spitsbergen) were terminated. Twelve percent of all buildings in Amderma are abandoned or in the state of emergency, while 32 of a total of 66 stone buildings are deformed. Only 2 of 19 thermal and power plants have insignificant deformations, and 1 are in an emergency state. The majority of buildings were constructed in the last 1 12 years; a significant number of the deformed old buildings are disassembled. The total number of deformed buildings (Table 2) is about 4% and above (Brouchkov, 1997; Velli, 198). Inspections of buildings in Amderma have shown that 18 of 268 buildings were deformed and cracked. 1. The common method consisting of tests on uniaxial compression by step loadings does not allow for the determination the bearing capacity of saline soils because of the creep characteristics; testing by constant loads is necessary; 2. The duration of tests should be at least 3 days for the estimation of the stress characteristics; 3. An increase in ice content of frozen saline soils leads to an increase in their bearing capacity; in some cases icy bases are preferable; 99

6 Table 4. Bearing capacity of frozen saline soils of marine type. Salinization Bearing capacity R, kpa (kg/cm 2 ), at temperature C (Depth of foundation 4 m) D sal, % Sands.5 7 (7.) 9 (9.) 11 (11.) 12 (12.).1 4 (4.) 5 (5.) 6 (6.) 75 (7.5).15 2 (2.) 35 (3.5) 45 (4.5) 55 (5.5).2 25 (2.5) 3 (3.) 4 (4.).3 2 (2.) 3 (3.).5 2 (2.) Silty sands.1 9 (9.) 1 (1.) 12 (12.) 135 (13.5).2 4 (4.) 55 (5.5) 9 (9.) 11 (11.).3 25 (2.5) 5 (5.) 7 (7.) 8 (8.).5 25 (2.5) 3 (3.) 4 (4.).8 2 (2.) Sandy silts and silts.2 45 (4.5) 7 (7.) 95 (9.5) 115 (11.5).5 15 (1.5) 3 (3.) 4 (4.) 75 (7.5) (1.5) 25 (2.5) 4 (4.) 4. For design calculations, it is necessary to take into account the long-term migration of salts in the saline frozen soils under temperature and salt content gradients. 6 CONCLUSIONS 1. Frozen saline soils are widely distributed along the Arctic coast and are characterized by low bearing capacity. Chemical composition of salts is important: frozen deposits of the marine type have the lowest strength. Ice normally plays a bearing role. 2. Salts change creep capabilities of soil: creep rate decreases with time, as confirmed by long-term experiments lasting 12 years. 3. Almost half of all buildings along the Russian Arctic cost have been damaged, and the major reasons for the damages were the following: underestimating of the influence of salinization of soils; uncontrolled heat emission due to water leaking and snow redistribution. Existing construction regulations concerning bearing capacity calculations of frozen saline soils need to be reviewed. ACKNOWLEDGEMENTS REFERENCES Aksenov, V.I. & Brouchkov, A.V Plastic Frozen (Saline) Soil as Base. Proceedings of the Sixth International Conference on Permafrost, Beijing, China: 1 5. Brouchkov, A.V Frozen saline soils of the Arctic coast, their origin and properties, Moscow: Moscow University Press. 33 p. (in Russian). Brouchkov, A.V. 2. Salt and water transfer in frozen soils induced by gradients of temperature and salt content. Permafrost and Periglacial Processes 11(2): Brouchkov, A.V. 22. Nature and distribution of frozen saline sediments on the Russian Arctic coast. Permafrost and Periglacial Processes 13(2): Hivon, E.G. & Sego, D.C Distribution of Saline Permafrost in the Northwest Territories, Canada. Canadian Geotechnical Journal, 3: Tsytovich, N.A The mechanics of frozen ground. New York: Scripta, McGraw-Hill, 426 p. Velli, Y.Y Foundations on complex permafrost soils, In: U.S. Army Cold Regions Research and Engineering Laboratory, SR 8-4 and Building under cold climates and on permafrost; collection of papers from a U.S.- Soviet joint seminar, Leningrad, USSR, Dec. 198: Vyalov, S.S Rheological foundations of soil mechanics. Amsterdam: Elsevier, 564 p. Williams, P.J. & Smith, M.W The Frozen Earth, Cambridge: Cambridge University Press, 36 p. I am grateful to the former staff members of Amderma permafrost station, Dr. V. Aksenov and Dr. Y. Velli for the assistance in the research. 1